1. Field of the Invention
The present invention relates generally to a portable radio system, a portable radio equipment to be used in the same, and a frequency error prediction method to be used therefor. More particularly, the invention relates to a frequency error prediction method in an automatic frequency control (AFC) of a portable radio equipment.
2. Description of the Related Art
In a portable radio equipment (mobile station) as a portable telephone or cellular phone of a system employing QPSK (Quadrature Phase Shift Keying) or WCDMA (Wideband Code Division Multiple Access), as a internal oscillator (mobile station oscillator) of the equipment, oscillators which are inexpensive and lower precision are frequently used for lowering cost.
Therefore, in the mobile station, a frequency shift of the internal oscillator is detected with reference to a reception wave transmitted from a base station having high frequency precision for feeding back to a internal oscillator to perform automatic frequency control (AFC) adapting to a frequency of the internal oscillator.
On the other hand, in order to reduce power consumption and circuit scale, method, such as CORDIC (Coordinate Rotation Digital Computation), is suitable.
In Japanese Unexamined Patent Publication No. Heisei 8-307157, there is disclosed a method for repeated estimation of a phase value and radius value on the basis of an initial in-phase signal and an initial Quadrature signal by using a CORDIC algorithm in a circuit determining the radius value and the phase value from the in-phase signal I(n) and the Quadrature signal Q(n).
In the foregoing portable radio equipment (mobile station), an oscillator which is inexpensive and low precision, is frequently used as an internal oscillator in the equipment for lowering cost. Therefore, automatic frequency control is required. At this time, frequency shift is derived in the following manner.
A phase, derived by multiplying one of two symbols (which are taken at a generation timing of the internal oscillator by receiving a known data, such as modulated pilot signal or the like in a base station,) by a complex conjugate of the other symbol, represents a phase difference of two symbols. In case of the signal, in which two symbols of the known signal are the same phase when the frequency of the internal oscillator is correct, the derived phase shift is divided by an interval of two signals to derive the frequency shift of the internal oscillator.
In the conventional frequency error predicting method, when CORDIC as known method is used in calculation of tan−1 (Arc Tangent) of derived phase shift, only ±π/2 can be detected.
In the radio mobile equipment, it is important to reduce a power consumption and size. When a phase calculator such as CORDIC or the like is employed in the radio mobile equipment, tap number of CORDIC is reduced, and error may be caused in derivation of the phase shift.
Upon calculating the frequency shift from the derived phase shift, the phase difference of two symbols used for detection of the phase shift is divided by the interval. Therefore, if the interval of the symbols becomes long, the frequency detection error becomes smaller even when the phase detection error is constant.
However, if the interval is initially set long, when the frequency error is sufficiently large and the absolute value of the phase shift becomes greater than π, correct frequency error cannot be detected.